This application relates to restoring capacity in a network following a failure of a network channel or node, particularly in a telecommunications network.
Telecommunications subscribers and telecommunications carriers generate a wide range of digital signals having a wide range of data rates. To carry this digital traffic, telecommunication carriers typically install high-capacity fiber optic links between switching offices and multiplex this digital traffic onto the fiber optic links. Synchronous Optical Network (SONET) is a standard for data transmission over fiber optic networks. SONET is widely used by telecommunications carriers in the United States, while a related standard (Synchronous Digital Hierarchy (SDH)) is used in some other countries. These standards define technologies for carrying many signals of different capacities through hierarchical synchronous optical networks. At the lowest level of such a hierarchy, these standards define transport signals. For example, SONET defines a Synchronous Transport Signal—level 1 (STS-1), which operates at a 51.84 Mbps data rate. Higher-level signals (STS-n) are integer multiples of STS-1. The optical counterpart of an STS-n signal is designated an Optical Carrier N (OC-n).
To provide highly reliable service, telecommunication carriers often create rings that interconnect a number of switching offices or other nodes. Each node is connected to the next node on the ring by an OC-n span. In a typical 4-fiber, bi-directional line switched ring (BLSR), four optical fibers extend between, and therefore interconnect, each pair of adjacent nodes. Each optical fiber provides one channel. Two of the fibers form two oppositely-directed, unidirectional “working” channels, and the remaining two fibers form two oppositely-directed, unidirectional “protect” channels. Each channel can carry traffic in either a clockwise or a counter clockwise direction along the ring from a source node to a destination node. The clockwise-oriented working channels collectively form a clockwise working ring. Similarly, the other channels form respective rings, i.e. a counter-clockwise working ring, a clockwise protect ring and a counter-clockwise protect ring.
The protect channels provide a backup capability, which can be used in case of a failure of a working channel, an entire span or a transit node. The protect channels can be used individually or collectively, depending on the failure. If only a working channel of a span fails, the like-directed (i.e., clockwise or counter-clockwise) protect channel along the same span can be used to take over for the failed working channel. A node (typically the node at the receiving end of an optical fiber) detects failures in the channels that terminate at that node. If the node detects a failure in an adjacent working channel or span, the node sends a command to the node at the opposite end of the failed span or channel (the “far node”) requesting that the far node connect (“bridge”) the appropriate working channel to the appropriate protect channel, thus switching network traffic onto the protect channel. This is referred to as “span switching.” Although only one working channel (clockwise or counterclockwise) in a span might have failed, typically both working channels are switched to their respective protect channels. This is commonly referred to as “bi-directional protection switching.”
If an entire span fails, such as a result of a cable cut or a node failure, all the protect channels of the rest of the ring take over for the failed span. In this case, the node that detects the failure requests the far node to bridge each of its working channels to the far node's oppositely-directed protect channel. The node that detects the failure also bridges its working channels to its oppositely-directed protect channels. Thus, the two nodes on opposite sides of the failure loop network traffic from their working channels onto oppositely-directed protect channels. The remaining nodes of the ring enter a pass-through mode, in which they bridge their like-directed protect channels (incoming clockwise protect channel to outgoing clockwise protect channel, etc.). Thus, a continuous path around the ring is restored, albeit a longer path that enters and leaves each of the remaining nodes twice. This is referred to as “ring switching.” Collectively, this self-restoring capability (including span switching and ring switching) is commonly referred to as automatic protection switching (APS).
STS-1 uses a 810-byte transport frame to carry payload and overhead information. The overhead information supports several “channels” (not to be confused with working and protect channels) that are used for operation, administration, maintenance and provisioning (OAM&P) of the network and higher level services. For example, the so-called K1 and K2 bytes provide an automatic protection switching (APS) channel, which enables line-terminating entities (LTEs) to send and receive remote defect indications (RDIs), alarm indication signals (AIS-L) and to implement the automatic protection switching discussed above. For example, nodes of a BLSR ring send commands via the APS channel to request other nodes to perform span switches or ring switches. These “bridge requests” include “forced switch-span” (FS-S) and “forced switch-ring” (FS-R).
Although SONET is widely used and robust, it has shortcomings. For example, BLSR rings are limited to a maximum of 16 nodes. Some telecommunication carriers would prefer to construct BLSR rings having more than sixteen nodes. Furthermore, the APS channel supports only sixteen unique bridge requests. Consequently, some BLSR functions, such as Clear, must be requested via “out-of-band” communication channels. A ring formed of a number of nodes and spans sometimes includes one or more “straddling” spans, i.e., spans that interconnect nodes of the ring but that do not lie along the ring. Although a theoretical framework for protecting straddling spans in pre-configured protection cycles (p-cycles) has been proposed, the BLSR protocol does not support protecting straddling spans.